As electronic and electric devices have been miniaturized in recent years, the mainstream of coils to be attached inside such devices is changing from one using a conventional enameled wire with a circular cross section (i.e., round enameled wire) to one using an enameled wire with a rectangular cross section (i.e., rectangular enameled wire). The rectangular enameled wire is made as a result that an insulating varnish is applied onto a conductor with a rectangular cross section (rectangular conductor) and baked, to form an insulating film. By using the rectangular enameled wire, a gap between the enameled wires when being wound into a coil can be made smaller (that is, a space factor of the enameled wire can be heightened), enabling miniaturization of the coil. Recently, in order to further miniaturize a coil, a diameter of an enameled wire is being made smaller.
The insulating film of the enameled wire used for the coil of the motor is required to have great flexibility, excellent heat resistance and heat deterioration resistance, and also have enough processing resistance to stand processing stress of being wound into the coil. In particular, a rectangle enameled wire with a small diameter is required to have higher processing resistance, as the processing stress is more severe. In addition, the rectangle enameled wire is required to be capable of maintaining higher voltage resistance even if it is used under a severe environment. In other words, it is required to have sufficient environmental atmosphere resistance. Such severe environments may include a high humidity and high temperature environment, an environment in which the enameled wire is in contact with oil such as an insulation oil, a machine oil, an engine oil, or a transmission oil, and/or water.
In order to satisfy the above-mentioned required characteristics, an insulated wire has been proposed in which an insulating varnish containing an adhesion improver such as highly-adhesive polyesterimide or highly-adhesive polyamideimide, is applied to a conductor and baked, and an aromatic polyamide film is formed in an outer periphery thereof. This insulated wire may have improved heat resistance and heat deterioration resistance. However, the insulated wire has little flexibility and insufficient processing resistance.
Therefore, the inventors of the present invention has conceived and developed an insulated wire in which layers of two kinds of polyamideimides with different characteristics, are laminated on the conductor, and the outermost layer of polyimider is provided. This insulated wire has great flexibility and enough processing resistance, as well as excellent heat resistance and heat deterioration resistance. However, the insulated wire still has slightly insufficient environmental atmosphere resistance. In addition, although the insulated wire has better processing resistance in comparison with the conventional insulated wire, it still leaves room for improvement in processing resistance, because the rectangle insulated wire with a small diameter is required to have higher processing resistance due to the fact that such a wire undergoes more severe processing stress when coiling.